In urban cities, most areas are brightly illuminated even at night by many light sources including automobile headlights and store illuminations. On the other hand, most areas in suburbs and provincial districts are dark at night, and therefore, streetlights and other outdoor luminaire are provided in areas such as pathways, roadways, parks, and tunnels for the safety and security purposes.
However, when excessively-high-intensity luminaires are installed in suburbs and provincial districts and turned on at night to produce bright illumination, the bright illumination may interfere in normal life for residents. Besides, it consumes excessive electrical power and requires high installation costs. It would be desirable to have outdoor luminaires which can attain the purposes of safety, security and crime prevention without excessive power consumption.
Recently, the light sources for streetlights and outdoor luminaires are considered from the standpoints of maintenance and economy, and LED lamps are of greater interest in view of advantageous in power consumption and lifetime over traditional incandescent lamps and fluorescent lamps. LED lamps have a wide variation of light emission. Among them, the LED lamps using the blue LED that produce light including a blue component may be used as the light source for outdoor luminaires, typically as the blue streetlights which are nowadays introduced as them. The power utility efficiency of LED lamps is very high. The power necessary for LED lamps to provide an equivalent illuminance is as low as ⅕ or less of the power consumed by incandescent lamps. LED lamps are generally believed to have a lifetime of several tens of thousands hours, leading to savings of labor and cost for maintenance service. For these reasons, LED lamps are suited for use in streetlights and outdoor luminaires which are intended for long-term continuous operation.
White LED packages for general lighting application have the structure that a phosphor emitting green, yellow, or red fluorescence is coated on the front surface of a blue LED chip having an emission wavelength of 440 to 470 nm. With this structure, broad luminescent light having a peak wavelength around 555 nm is combined with blue light of unconverted wavelength, thereby white light is produced. White LED lamps for most luminaires utilize luminescent light having a peak wavelength of about 555 nm because human eyes have high sensitivity to light of wavelength near 555 nm. In principle, light rich in such wavelength components provides highly effective lighting. However, it is known that under scotopic vision conditions at night streets or under mesopic vision conditions immediately after sunset, the peak wavelength of human visual sensitivity shifts from 555 nm to the shorter wavelength side as shown in FIG. 7. This is known as the Purkinje effect.
Under scotopic and mesopic vision conditions, the night luminaires using white LEDs having an emission peak shifted to a wavelength shorter than 555 nm are preferred. For night luminaire, luminaires having a high color temperature are under study (Non-Patent Document 1). These prior art night-lighting luminaires are not regarded as complying with a change of visual sensitivity based on the Purkinje effect because the light does not fully contain wavelength components of the highest visual sensitivity at the scotopic and mesopic vision levels.
Photoreceptor cone cells and rod cells are distributed in the human eyeball retina, and it has been known that the cone cells are responsible for color vision in bright places while the rod cells are responsible for sensing the light in dark places. These cone cells and rod cells are not evenly distributed on the human retina, and the cone cells are densely packed in the part called “fovea centralis”, and the rod cells are concentrated at the outer edges of the retina. In other words, human eyes are conceived to have a structure such that brightness is sensed by peripheral vision rather than the central vision in the environment such as scotopic and mesopic vision conditions where the rod cells are the main contributor. In view of such situation, the lighting apparatus used in such environment preferably is the one having higher light amount in the peripheral area remote from the optical axis. However, most of the LED night luminaires are brightest in the area of the optical axis with the light amount rapidly reducing toward the periphery. By the way, the LED luminaires recently used for streetlights often have high color temperature. However, these streetlights are merely designed to increase the quantity of blue light without taking into account the visual sensitivity under scotopic and mesopic vision conditions.